Third Generation (3G) Networks such as UMTS (Universal Telecommunication Network) and CDMA 2000 provide high-speed wireless Internet access to mobile users over a wide coverage area. For the 3G networks the IP Multimedia Subsystem IMS has been defined to provide cellular access to the services of the Internet in order to support telephony and multimedia services. The IMS uses packet-based technology, in particular IP-network and other IETF protocols for provision of services. The strength of IMS is the provision of enhanced services, for example multimedia services combining voice and data. Further, the usage of IP-network as a single underlying standard allows an easy and fast service deployment. In contrary, 2nd Generation networks, like GSM, provide voice based on a circuit-switched technology.
Besides voice and data calls, in UMTS 3GPP standard a number of so-called mid-call services are defined, which are characterised by the fact that during an ongoing call the user initiates a service invocation. Some examples of mid-call services are:                Communication Diversion/Call Deflection (CDIV/CD)        Explicit Communication Transfer (ECT)        Communication Hold/Resume/Retrieve (HOLD)        Conference and 3 party call (MPTY, CONF)        Call Waiting (CW)        Call Completion to busy Subscribers (CCBS)        
A detailed description of the different mid-call might be found in 3GPP TS 22.004 for TS11 and other teleservices.
A Session Initiation Protocol SIP has been chosen in IMS for signalling between the user's equipment UE and the IMS as well as between the components within the IMS. The IMS uses SIP also to complete voice and multimedia calls in the Internet. In order to be able to use the IMS service, the communicating user's equipment has to support IMS, which means SIP has to be implemented in the user's equipment.
The IMS supports inter-working with legacy networks. This support is provided by a Media Gateway Control Function (MGCF). The MGCF performs protocol conversion between cellular call control protocols and IMS protocols. For example, the MGCF receives a SIP message from the IMS and converts it into appropriate ISUP messages. Thus, the primary function of MGCF is to convert signalling information from one format to another in uplink and downlink direction.
The IMS has been deployed for 3G networks for provision of services using packet-based technology with SIP as applied signalling protocol. IMS provides, amongst others, Call State Control Functions (CSCF), which allow routing of SIP messages from and to users connected via access networks to application servers and other control functions. There are three types of CSCF, namely Proxy-CSCF (P-CSCF), Interrogating-CSCF (I-CSCF) and Serving-CSCF (S-CSCF). IMS services are implemented in application servers. One main task of the P-CSCF is to store the S-CSCF address for the user, the I-CSCF supports the lookup of the S-CSCF for a user' call and is a contacting point providing a service session control.
However, currently the major number of user's equipment do not support IMS technology with SIP as signalling protocol for voice and mid-call services, since said user's equipment is adapted for a circuit-switched controlled domain. Thus, for the access to the IMS an adaptation of the user's equipment is necessary.